In the chopper controllers of electric cars, high-frequency electric currents generated by the chopping operation cause an effect as a noise source for several kinds of traffic signal equipment. Therefore, it is desirable that the chopping frequency be selected such that it and its harmonics do not fall within the frequency region of the signal of such equipment. However, the frequency of chopper control signal to be supplied to the chopper controller may change or deviate due to some causes, such as an abnormal condition developed in the oscillator circuit that generates the chopper control signal. If such a frequency deviation causes the higher harmonics of the chopping frequency to fall within the frequency region of the traffic signals that are being used for the signal equipment, the chopping frequency having large energy is interrupted into the signal equipment to cause them to operate erroneously. Such a state should, therefore, be detected quickly. Namely, means should be provided which monitors the frequency of the chopper control signal, and which produces an alarm in case the frequency deviates by more than a predetermined value.